The present invention relates to novel fluorine-containing polyfunctional (meth)acrylate; monomer compositions which can be used as a starting material for preparing a low refractivity material which has both high surface hardness and low refractive index and which can be applied to the surface of various kinds of substrates; a low refractivity material prepared by curing the monomer composition by polymerization; and a reflection reducing film provided with the low refractivity material.
Compounds having a fluorine atom have low refractive index, and can be used for antireflection films or a clad material for optical fibers. In either applications, the lower the refractive index of the compound, the better the property of the products. There are proposed, for example, application of fluorine-containing (meth)acrylate polymers, copolymers of fluorine-containing (meth)acrylate with other monomers, tetrafluoroethylene polymers, copolymers of vinylidene fluoride and tetrafluoroethylene, or copolymers of vinylidene fluoride and hexafluoropropylene to optical fibers (Japanese Laid-open Patent Application Nos. 59-84203, 59-84204, 59-98116, 59-147011, and 59-204002).
Recently, there has attempted to apply solvent-soluble fluorine-containing polymers having low refractive index such as fluoroalkyl acrylate polymers, fluoroalkyl methacrylate polymers, or amorphous perfluoro resins such as CYTOP (trade name) manufactured by ASAHI GLASS COMPANY, or TEFRON AF (trade name) manufactured by E.I. du Pont de Nemours and Co. to reflection reducing films (Japanese Laid-open Patent Application Nos. 64-16873, 1-149808, and 6-115023).
These fluorine-containing resins, however, are non-cross-linked resins, and thus have low surface hardness after curing, inferior abrasion resistance, and insufficient adhesion.
For the purpose of improving the surface hardness, there has been proposed cross-linked polymers prepared from a suitable mixture of fluorine-containing monofunctional (meth)acrylate or fluorine-containing bifunctional (meth)acrylate and polyfunctional (meth)acrylate not containing fluorine (Japanese Laid-open Patent Application Nos. 58-105943, 62-199643, and 62-250047). The refractive index and the surface hardness of these cross-linked polymers may be adjusted to some extent by suitably selecting the content of fluorine in the fluorine-containing (meth)acrylate, or the mixing ratio of the fluorine-containing (meth)acrylate to the polyfunctional (meth)acrylate not containing fluorine. However, the fluorine-containing monofunctional (meth)acrylate and the polyfunctional (meth)acrylate are not compatible, and do not dissolve mutually at an arbitrary ratio. Therefore, sufficiently low refractive index cannot be achieved. On the contrary, the fluorine-containing bifunctional (meth)acrylate and the polyfunctional (meth)acrylate mutually dissolve at an arbitrary ratio. However, if the content of fluorine atoms in the cross-linked polymer is increased for reducing the refractive index, the cross-linking density is lowered. Accordingly, it is impossible to suffice both the low refractive index and the high surface hardness, and it is difficult to give sufficient surface hardness to the optical fibers and the reflection reducing film. Further, sufficient adhesion cannot be achieved.
There is also proposed fluorine-containing hydroxy (meth)acrylate for the purpose of improving the adhesion and for use as a starting material for other fluorine-containing (meth)acrylates (Japanese Laid-open Patent Application Nos. 4-321660, 4-356443, and 4-356444). However, since these compounds are monofunctional (meth)acrylate, the surface hardness after curing is low, and the abrasion resistance is inferior.